1. Field of the Invention
The present invention relates to a display apparatus which can be applied to a device which displays a video image in superimposed fashion on a background view of the outside world which can be seen from a display region having transmissive properties, whereby the video image and the background view of the outside world are viewed together, and to a mobile object having the display apparatus, and a control apparatus which controls the image display unit.
2. Description of the Background Art
A driver who is driving a vehicle, such as an automobile, is required to perform tasks such as ascertaining the circumstances outside the vehicle while driving, reading information on a display apparatus of the vehicle, and carrying out driving operations, and the like, safely and quickly, and therefore desirably the driver is able to read out information on the display apparatus of the vehicle without the range of movement of the driver's viewpoint leaving the range required to ascertain the circumstances outside the vehicle while driving. Consequently, an image display apparatus is anticipated, which displays text characters or images by irradiating light onto a portion of a transparent sheet, such as the windshield of the vehicle, for instance.
Possible examples of a transmissive display apparatus of this kind are a head-up display which displays driving information on the windshield of an automobile (hereinafter, called HUD), and a head-mounted display which displays information on lens portions of eyeglasses (hereinafter, called HMD), and the like. When a transmissive device of this kind is used, a driver is able to see information relating to driving (for example, a map or speedometer) while simultaneously viewing the outside world, and therefore it is expected that the driver is able to drive more safely.
A conventional HUD is one which projects a virtual image onto a windshield. FIG. 28 shows an example of a conventional HUD. In this example, it is possible for a driver to see a view of the outside world and to view display light, simultaneously, by reflecting the display light toward the driver in a portion of the windshield.
In FIG. 28, 101 denotes a vehicle body in which the HUD is installed. 102 denotes an HUD optical unit which is accommodated inside a dashboard, and which comprises a display unit 103 and a deflection unit 104 inside. The display unit 103 consists of a liquid crystal element and a light source, for example, and displays information that is to be displayed to the driver 107. The display light displayed on the display unit 103 is projected toward the deflection unit 104. The deflection unit 104 is constituted by a mirror, or the like, and deflects the display light from the display unit 103 toward the windshield 106. The display light deflected by the deflection unit 104 is projected onto the windshield 106 by passing through an optical system aperture section 105 of the HUD optical unit 102. The windshield 106 reflects the display light from the HUD optical unit 102 toward the driver 107. The driver 107 is able to view information relating to driving operations, by looking at the display light reflected by the windshield 106. An eyebox 108 indicates the range in which the image is visible when the driver 107 moves his or her head, and when the eyes of the driver 107 are inside the eyebox 108, the display light reaches the retinas of the driver 107 and the driver 107 is able to see the image. Furthermore, the windshield 106 transmits light from outside (outside world) of the windshield 106, simultaneously with reflecting the display light from the HUD optical unit 102. Due to these characteristics of the windshield 106, the driver 107 is able to see a view of the outside world of the windshield 106, together with the information displayed by the display unit 103. In this way, in an HUD, it is possible for a driver 107 to view information from the display unit 103 without the driver's line of sight moving away from the outside world, and therefore the requirement for the driver 107 to move his or her line of sight is reduced and the safety during driving is improved.
In the present specification, the ratio of light from outside the windshield 106 which reaches the interior of the vehicle by passing through the windshield 106 is called the outside world transmittance. The higher the outside world transmittance, the more clearly the driver 107 is able to see the view outside the vehicle.
However, in a conventional HUD which reflects display light at the windshield 106 in this way, there is a problem in that a double image is produced. This example is shown in FIG. 29. FIG. 29 is a diagram showing the composition of the windshield 106, in which an intermediate film 201 is disposed between an inner glass 202 and an outer glass 203 which are two sheets that constitute the windshield 106. Incident light 204 which is incident on this windshield 106 is reflected by both the front surface of the inner glass 202 and the rear surface of the outer glass 203, as shown in FIG. 29, thereby giving rise to front surface reflected light 205 and rear surface reflected light 206. In this case, since the light path of the front surface reflected light 205 and the light path of the rear surface reflected light 206 are different due to the thickness of the glass, the driver 107 sees a double image when the driver views the front surface reflected light 205 and the rear surface reflected light 206 at the same time. This phenomenon is called a double image. FIG. 30 shows an example of a double image. In FIG. 30, 301 indicates an image which is seen by a driver 107 due to the front surface reflected light 205. Similarly, 302 indicates an image which is seen by the driver 107 due to the rear surface reflected light 206. As shown in FIG. 30, since the display image 301 and the reflected image 302 have different display positions, the image seen by the driver 107 appears double as shown in FIG. 30. In the present specification, the problem caused by a driver 107 simultaneously viewing reflected light 205 from the front surface of the inner glass 202 and reflected light 206 from the rear surface of the outer glass 203 is called a double image problem.
The method for solving this double image problem may be a method which provides a narrow band filter on the front surface of the windshield (see Japanese Patent Application Publication No. S58-181004, for example). Here, the narrow band filter indicates a filter which reflects, with a high reflectivity, only light of a particular wavelength range, and transmits light of other wavelengths with a high transmissivity. FIG. 31 shows an example of the reflection characteristics of a narrow band filter. FIG. 31 shows the reflectivities of light of respective wavelengths when light is incident at a design incidence angle θa, in which light of a wavelength included in a reflected wavelength range 501 is reflected with a reflectivity of R, and light of other wavelengths is transmitted without being reflected. To give a specific example of a narrow band filter, it is possible to use a multi-layered film filter which is composed by layering together a material of low refractive index and a material of high refractive index or rugate filter, and the reflectivity R can be raised by using a filter of this kind. In the present specification, the “reflected wavelength range” is a range of wavelengths showing a reflectivity of no less than 80% with respect to a peak reflectivity R, for instance.
FIG. 32 shows an example of preventing the occurrence of a double image by using a narrow band filter of this kind. In FIG. 32, a narrow band filter 401 is disposed on the front surface of the windshield 106, on the interior side of the vehicle. In this case, the reflected wavelength region which is reflected by the narrow band filter 401 is designed so as to coincide with the wavelength range included in the incident light 204, and the incident light 204 is reflected rather than being transmitted by the narrow band filter 401. In this case, since the incident light 204 does not reach the outer glass 203, reflection does not occur at the interface between the outer glass 203 and the air (the rear surface), and the driver sees only the front surface reflected light 205. It is possible to solve the double image problem by using a narrow band filter 401 in this way.